US20060114970A1 - Method and apparatus for performing signal correlation at multiple resolutions to mitigate multipath interference - Google Patents

Abstract

A method and apparatus for selecting a signal processing resolution. A received signal is processed using a first resolution. When a multipath condition is detected, the signal is processed using a second resolution.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This application is a continuation of co-pending U.S. patent application Ser. No. 10/218,110, filed Aug. 13, 2002, which is a continuation in part of U.S. patent application Ser. No. 09/900,499, filed Jul. 6, 2001 (now U.S. Pat. No. 6,704,348, issued Mar. 9, 2004, which is a continuation in part of U.S. patent application Ser. No. 09/861,086, filed May 18, 2001 (now U.S. Pat. No. 6,606,346, issued Aug. 12, 2003). Each of the aforementioned related patent applications is herein incorporated by reference.
BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• The present invention relates to signal correlators for digital signal receivers and, more particularly, the invention relates to a method and apparatus for performing signal correlation in, for example, a global positioning system (GPS) receiver.
• 2. Description of the Related Art
• The process of measuring a global positioning system (GPS) signal begins with a procedure to search for the GPS signal in the presence of noise by attempting a series of correlations of the incoming signal against a known pseudo-random noise (PRN) code. The search process can be lengthy, as both the exact frequency of the signal and the time-of-arrival delay are unknown. To find the signal, receivers traditionally conduct a two dimensional search, checking each delay possibility at every possible frequency. To test for the presence of a signal at a particular frequency and delay, the receiver is tuned to the frequency, and the incoming signal is correlated with the known PRN code delayed by an amount corresponding to the time of arrival. If no signal is detected, the search continues to the next delay possibility, and after all delay possibilities are checked, continues to the next frequency possibility. Each individual correlation is performed over one or more milliseconds in order to allow sufficient signal averaging to distinguish the signal from the noise. Because many thousand frequency and delay possibilities are checked, the overall acquisition process can take tens of seconds.
• Recently, new applications of GPS technology in wireless devices have emerged, for example, the use of GPS in cellular phones to provide emergency location capability. In these applications, rapid signal acquisition in just a few seconds is required. Furthermore, these applications require a GPS receiver to operate in harsh signal environments and indoors, where GPS signal levels are greatly attenuated. Detecting attenuated signals requires each correlation to be performed over a relatively long period of time. For example integration may be performed over a few seconds, as opposed to the 1-10 millisecond period used in traditional GPS receivers. The two dimensional sequential search process employed by traditional receivers becomes impractical at such long integration times, because the overall search time increases by a factor of 100 or more.
• To accelerate the search process, GPS designers add additional correlators to the receiver so that multiple time of arrival possibilities can be checked simultaneously. Typically, each correlator that is added requires a separate code mixer and signal accumulator. For a given sensitivity level, this decreases search times in proportion to the number of correlators. To achieve the sensitivity and acquisition time demanded in cellular phone applications, the design might have to incorporate thousands of correlators. This addition is typically prohibitively complex and expensive for a consumer class device.
• For example, U.S. Pat. No. 5,901,171, issued May 4, 1999, describes a triple multiplexing technique that allows a single time shared processing block to be used to perform up to 20 simultaneous correlations on each of 12 channels. This offers an improvement in performance relative to single correlator designs since blocks of 20 delay possibilities are checked simultaneously. A full signal search over a full range of delay uncertainties requires using the block of 20 correlators approximately 100 times in succession to check 2046 delays. Thus, if an acquisition must be performed in a few seconds, the integration time is limited to tens of milliseconds. This is insufficient to achieve the sensitivity needed for indoor GPS applications.
• To further improve the search process, other GPS receiver architectures include processing capable of generating a convolution between the incoming signal and the known PRN code. This is equivalent to providing a complete set of correlators spanning all time delay possibilities over a full C/A code epoch (1023 chips), and U.S. Pat. No. 5,663,734, issued Sep. 2, 1997, describe fast Fourier transform (FFT) based software techniques to efficiently generate the necessary correlation results using software algorithms. This approach is not suitable for all applications, because a programmable digital signal processor (DSP) is needed to run the software FFT, and a large memory is needed to store unprocessed signal samples. Furthermore, this approach can have a large processing delay due to the software computations and the fact that software processing starts only after a complete snapshot of the signal is stored. In many applications, a real time processing solution is preferred, preferably one that does not involve extensive software processing. Lyusin et al., “Fast Acquisition by Matched Filter Technique for GPS/GLONASS Receivers”, pp 307-315 describes hardware approaches to performing the convolution in real time using a matched filter with 1023 taps. The matched filter consists of shift registers large enough to hold a full C/A code epoch, as well as awidth 1023 vector multiplier and adder unit that generates the inner product between a full epoch of the signal and the C/A code.
• This circuit is complex relative to the constraints of low cost consumer devices such as cellular phones. Other matched filter approaches, such as utilized in military class receivers for P-code acquisition, also incorporate large vector multipliers.
• Thus, there is a need for an improved, simple and low cost GPS processing block capable of processing an entire epoch of signal and C/A code. Such a device must be built from hardware of relative simplicity, yet be capable of generating a full convolution, or many parallel correlations, preferably without a large vector multiplier.
SUMMARY OF THE INVENTION
• The present invention is a method and apparatus for mitigating multipath interference in a digital signal correlator. The present invention forms values representative of magnitude or power of a correlation response between a digital signal and a pseudorandom reference code. The present invention then selects an earliest peak in the correlation response to compute a time delay. In one embodiment, the present invention selects the earliest peak in the correlation response by determining a plurality of maxima for the correlation response. The present invention then selects the earliest maximum in the plurality of maxima as the earliest correlation peak.
• In another embodiment, the present invention computes the slope of an ascending edge of the correlation response. The present invention also receives the mean noise level of the correlation response. The invention uses the intersection of the slope and the mean noise level to determine the earliest correlation peak in the correlation response.
BRIEF DESCRIPTION OF THE DRAWINGS
• So that the manner in which the above recited features of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings.
• It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
• FIG. 1 shows a block diagram of a GPS receiver comprising the present invention;
• FIG. 2 shows an example of waveforms produced by the invention ofFIG. 1;
• FIG. 3 shows details of an accumulated magnitude waveform ofFIG. 2;
• FIG. 4 shows a detailed block diagram of one embodiment of the convolution processor and the convolution results processing circuits;
• FIG. 5 depicts a flow diagram of a method of operation of the invention ofFIG. 1;
• FIG. 6 graphically illustrates a simplified example of computing a full convolution in the traditional manner;
• FIG. 7 graphically illustrates how the full convolution ofFIG. 6 is performed using the invention;
• FIG. 8 illustrates an embodiment of a code lookup apparatus suitable for use in the invention ofFIG. 1;
• FIG. 9 illustrates an embodiment of a two-dimensional code shift register suitable for use in an alternate embodiment of the invention ofFIG. 1;
• FIG. 10 shows a block diagram of a GPS receiver comprising an alternative embodiment of the invention;
• FIG. 11 shows details of an accumulated magnitude waveform in high resolution mode;
• FIG. 12 illustrates an embodiment of a code lookup apparatus suitable for use in the invention ofFIG. 10;
• FIG. 13 depicts a flow diagram of a method of operation of the invention ofFIG. 10;
• FIGS.14A-B show the details of correlation responses affected by multipath signals;
• FIG. 15 depicts a flow diagram showing a method for mitigating multipath interference in a digital signal correlator.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
• FIG. 1 depicts a block diagram of a global positioning system (GPS)receiver100 incorporating the present invention. The use of a GPS receiver as the platform within which the invention is incorporated forms one application of the invention. Other platforms that require signal correlation may find use for the present invention.
• Signals (such as GPS signals) are received by an antenna101. A radio-frequency-to-intermediate-frequency converter (RF/IF converter)102 filters, amplifies, and frequency shifts the signal for digitization by an analog-to-digital converter (A/D)103. Theelements101,102 and103 are substantially similar to those elements used in a conventional GPS receiver.
• The output of the A/D103 is coupled to a set of processing channels104₁,104₂, . . .104_n(where n is an integer) implemented in digital logic. Each processing channel104_nmay be used to process the signal from a particular GPS satellite. The signal in a particular channel is tuned digitally by atuner105, driven by a numerically controlled oscillator (NCO)106. Thetuner105 serves two purposes. First, the IF frequency component remaining after RF/IF conversion is removed. Second, the satellite Doppler frequency shift resulting from satellite motion, user motion, and reference frequency errors is removed. The output from the tuner is a baseband signal consisting of an in-phase component (I) and a quadrature component (Q). Thetuner105 andCarrier NCO106 are substantially similar to those used in conventional GPS receiver designs.
• Adecimation circuit107 processes the output of thetuner105. The output of thedecimation circuit107 is a series of complex signal samples with I and Q components, output at a rate precisely timed to match the timing of the input signal. In one embodiment of the invention, the decimation operation is a simple pre-summer that sums all the incoming signal samples over the period of an output sample. A numerically controlled oscillator (NCO)108 is used to time the sampling process. For example, if P=2, thecode NCO108 is set to generate a frequency of (2×f_s), where f_sis f_o(the GPS signal's C/A code chipping rate), adjusted for Doppler shift. The NCO adjusts for Doppler shift based on external input from firmware commands. Because the Doppler shift is different for each satellite, aseparate code NCO108 anddecimation circuit107 is required for each channel104_n. It should be noted that there is no requirement that the incoming sample rate be an integer multiple of the f_s, as thecode NCO108 is capable of generating an arbitrary frequency. If thedecimation circuit107 is a pre-summer, the number of samples summed will typically toggle between two values, so that over the long term, the correct sample timing is maintained. For example, if the incoming sample rate is 10 MHz, and the desired sample rate is 2.046 MHz, the pre-summer will add either 4 or 5 samples, so that the desired sample rate is maintained on average.
• Thedecimation circuit107 may also include a quantizer (not shown) at its output to reduce the number of bits in the signal components before further processing. In one embodiment of the invention, 2-bit quantization is used.
• The signal samples fromdecimation circuit107 are coupled to aconvolution processor109. Theconvolution processor109 generates results that are stored in signal random access memories (RAMs)110aand110b. Specifically, theseRAMs110aand110bhold a complex vector that makes up all or part of the full convolution between the input signal and a reference PN code (e.g. a GPS C/A code). The convolution result will have a peak at points corresponding to high correlation between the signal and reference (the PN code). As shall be discussed in detail below, the relative location of these peaks for various satellite signals is used to ultimately compute position information.
• Theconvolution processor109 and signalRAMs110aand110baccumulate convolution results for multiple epochs of the GPS signal, which repeats at nominal 1 millisecond intervals. For example, if 10 milliseconds of the signal are processed, the values inRAM110aand110bare the sum of 10 correlation results each generated over one epoch. All the individual correlations should have a similar characteristic, since the timing of the decimation operation ensures that samples are taken at the same relative moment within each epoch. Accumulating similar results from individual correlations improves the signal to noise ratio, enhancing the ability of the receiver to detect weak signals. This processing may be referred to as coherent integration and, as will be discussed, can be combined with magnitude integration to yield correlation results averaged over a time period of up to several seconds.
• The length of time over which coherent integration interval is performed is limited by several factors, including uncompensated Doppler shift, GPS signal navigation data bits, and phase shifts induced by motion of thereceiver100. These factors introduce slow, but seemingly random phase variations into the signals. Over many tens of milliseconds, these phase changes cause destructive interference that defeats the purpose of coherent integration. Therefore, to achieve long averaging intervals, thereceiver100 performs a secondary step of magnitude accumulation. Specifically, the signals stored in thesignal RAMs110aand110bare periodically output to acomplex normalizer111 that generates a complex magnitude value of the complex convolution vector. The complex magnitude values are accumulated by anadder112 and stored inmagnitude RAM113. Each time the complex magnitude of the signal is computed, thesignal RAMs110aand110bare cleared to allow another coherent integration to occur. The process continues until the desired number of magnitude accumulations is completed. For example, if the coherent averaging interval is 10 milliseconds, and 200 magnitude accumulations are desired, the total process will run over 2 seconds.
• After convolution processing, themagnitude RAM113 contains a vector containing the complex magnitude of the convolution result, integrated to improve signal-to-noise ratio. As shall be discussed below, this vector is further processed by software algorithms that are executed by theCPU114 to produce pseudorange data that is used to yield the position of the receiver. It should be noted that the CPU computational load for these steps is quite modest compared to a conventional GPS receiver or an FFT based correlator. In this implementation, the computationally intensive tasks of correlation and integration are completed prior to software processing.
• FIG. 2 depictswaveforms201I,201Q and202 generated by the components ofFIG. 1. Thewaveforms201I,201Q and202 are plots of signal strength (axis208) versus code chips (axis210). The waveforms depict the output of theconvolution processor109 during coherent integration and magnitude integration. For clarity, only 9 milliseconds of signal processing time is shown consisting of 3 magnitude accumulations each based on 3 coherent integrations. In the example, P=2, so there are 2046 signal samples per coherent integration.Waveforms201I and201Q are the output from theconvolution processor109 where201I is the I-component of the output and201Q is the Q-component. Each block of 2046 samples is a full convolution result, generated in real time by theconvolution processor109 from the 2046 signal samples processed during the interval. The convolution result contains noise except in the vicinity of a single peak (such as indicated byreference numbers206I and206Q) corresponding to the time delay of the signal. The signal repeats every epoch, so the peak reappears each 2046 samples. Over the first three cycles, correlation results are accumulated in theRAM110aand110bby summing values at corresponding delays from each epoch. (For example, the values atoutput time4 are summed with the values at output time2050 and4096.) The correlation peak always appears at the same delay offset and the size of the peak increases over the accumulation, roughly tripling over 3 epochs. The level of the noise also increases, but rises only as the square root of 3 because the noise correlation is uncorrelated from epoch to epoch. The signal to noise ratio improves through the accumulation process, increasing by roughly the square root of 3.Waveform201Q illustrates the same signal accumulation process occurring in the quadrature channel.
• Beginning with the 4^thcycle of the signal, thesignal RAMs110aand110bare cleared to zero, and the signal accumulation process begins again.Waveforms201I and201Q show the correlations accumulating and dumping 3 times over 9 signal epochs.
• At the end of the coherent averaging interval the accumulated signal's magnitude is computed and summed into themagnitude RAM113. The signal in themagnitude RAM113 is shown aswaveform202. In the example, thewaveform202 updates three times corresponding to the completion of each coherent integration. The peaks are identified by reference numbers212₁,212₂,212₃and noise is identified byreference number214. As can be seen, the signal-to-noise ratio increases with each magnitude accumulation, further enhancing the ability of the system to identify the peak corresponding to the time of arrival.
• It should be noted that in the example, the complex phase of the signal varied over the 9 epochs. In particular, the signal was initially present in both I and Q channels, but by the final epoch, had rotated so that the signal was strong in the I channel and nearly absent in the Q channel. As mentioned above, imperfect Doppler shift tuning and other effects cause this rotation. Over many epochs, the phase would rotate through many cycles, resulting in cancellation of the signal when accumulated. For this reason, the inventive receiver accumulates coherently over only a short interval, relying on magnitude (non-coherent) accumulation for long term averaging. Magnitude values are independent of phase, and may be successfully integrated over several seconds.
• FIG. 3 illustrates the accumulatedmagnitude waveform202 in greater detail. Theplot300 shows the magnitude of the convolution in the vicinity of a peak212₃corresponding to the time delay of the signal. Points on thecode chip axis210 are spaced at an interval equal to the C/A code chip length divided by P, where P is the ratio of the signal sampling rate to f_o, the C/A code chipping rate. In the example, P=2, so the points are spaced at ½ chip intervals, or approximately 500 ns. (This spacing in time corresponds to a range difference of 150 meters). In order to achieve pseudorange measurements on the order of ten meters or better, the convolution results are further processed, typically in theCPU114, to produce the position information. There are numerous interpolation techniques that can be used to estimate the true time delay, using the discrete correlation values provided by the convolution process. One embodiment uses a least squares estimation technique to identify parameters of a signal that best fits the noisy measured data. The ideal response of a signal is the magnitude of the autocorrelation of the signal. This waveform can easily be shown to have the form of a raisedtriangle302. Thewidth303 of the triangle base is exactly 2 C/A code chips, or 4 points on the convolution result (for the P=2 case). The height304 of the base of the triangle is the magnitude of the noise in the convolution for time delays not corresponding to the signal. The magnitude of this noise can be estimated from the data or pre-calculated based on design parameters, such as the amplifier noise figure, cable and filter loss and system temperature. Thepeak305 of the triangle and thecenter306 of the triangle are unknowns corresponding to the signal magnitude and time delay. The least squares method can be used to estimate these two parameters so as to fit the noisy data points to a triangle with a given peak and center.
• FIG. 4 depicts a detailed block diagram of the convolution processor109 (as well as the convolution results processing circuits400), in particular details showing how a full convolution is generated by repeated use of a small block of circuitry. Operation of the circuits in this embodiment can be best understood with simultaneous reference toFIG. 4, a flow diagram ofFIG. 5 representing the operation of theprocessor109 ofFIG. 4, and by comparison of the simple examples ofFIG. 6 andFIG. 7.
• Signals from thedecimation circuit107 are coupled to shiftregisters401aand401b, handling I and Q components, respectively. Eachshift register401aand401bis of length P×K, where P is the desired number of samples per C/A code chip, and K is chosen as a design parameter. As will be explained K is a factor of 1023. To simplify the discussion, the remainder of the discussion focuses on one particular embodiment with P=2 (samples spaced ½ chip apart) and K=33. This means of advancing the signal through the shift register eliminates the need for circuitry to double-buffer the signal, reducing the cost and complexity of implementation.
• Signals advance throughshift registers401aand401bat the rate of 2f_o, as timed by thecode NCO108. The signals remain in place in the shift registers for many clock cycles, so that a series of partial correlation operations can be performed. Specifically, a total of M partial correlations are performed, where M=1023/K or 31 in this example. Each partial correlation consists of a fast vector multiply and add operation between the contents of each signal shift register and a segment of the code containing P×K (e.g., 66) code samples. The fast vector multiplication and addition occurs incircuits402aand402b.Circuits402aand402brespectively comprisemultipliers410aand410bandsummers412aand412b. The operation consists of multiplying each of the 66 signal samples in the signal register401aor401bby 66 code samples (formed by extending 33 code samples with the code extender409), and summing the results insummer412aand412b. The operation occurs separately and simultaneously in the I and Q channels. Mathematically, this operation is referred to as an inner product, defined as$\sum _{i=1}^{P\times K}\langle {\mathrm{signal}}_i\rangle \langle \mathrm{code}{c}_i\rangle$
  The output of the vector multiply and add may be re-quantized to keep the numbers in a small range so as to avoid overflowing RAMs404aand404b. For simplicity, the quantizer is not shown. In one embodiment, the re-quantization is to 2 bits of resolution.
• The results of the vector multiply and add are accumulated byadders403aand403band processed by the convolutionresults processing circuits400.Circuits400 comprisesignal RAM110a,110b,complex normalizer111,adder112 andmagnitude RAM113. The accumulation process consists of reading fromRAM110aand110bthe current values for a particular time delay, adding the just computed partial correlations, and writing the sums back toRAMs110aand110b. By properly combining partial correlations that correspond to a particular time delay, the full correlation for that delay is computed. As described previously, the process continues for as many epochs of the signal as desired to enhance signal to noise ratio. Thus, theadders403aand403bserve two purposes: the combining of partial correlations within an epoch; and the accumulation of correlations across several epochs.
• The outputs fromsignal RAMs110aand110bare combined in complex normalizer405 to form the magnitude of the signal. The I and Q waveforms in theseRAMs110aand110bcan be viewed as the real and imaginary part of a complex waveform. Forming the magnitude consists of squaring each component, summing the results, and taking the square root of the result. There are several approximations to the magnitude that can be used to simplify circuitry. In one embodiment, the complex magnitude is approximated by taking the scalar magnitude of I and Q independently and determining which is larger. The magnitude can be approximated by taking the larger magnitude and adding it to the one half of the smaller magnitude.
• The results of the magnitude operation may be scaled to keep the values in a small range so as to avoid overflowingRAM113. For simplicity, a scaler is not shown. In one embodiment, the scaling consists of shifting the result by 3 bits (i.e., divide by 8).
• It would also be possible to accumulate signal powers rather than signal magnitudes. In this case, the operation in405 would be power estimation, typically computed by taking the sum of the squares of I and Q. In this case, the pseudorange determination algorithms described in reference toFIG. 3 would have to be slightly modified to perform a fit against a power waveform as opposed to a magnitude waveform. Alternatively, additional nonlinear operations could be used to generate values representative of the magnitude or power of I and Q.
• The output fromcomplex normalizer111 is accumulated by theadder112 intomagnitude RAM113. The accumulation process consists of reading fromRAM113 the current magnitude value for a particular time delay, adding in the just computed magnitude result, and writing the sum back to theRAM113. As discussed previously, the magnitude accumulation continues for as many cycles as required to achieve signal to noise ratio enhancement.
• Thevector multipliers402aand402bperform M partial correlations for each shift of the signal. Acode lookup circuit408 generates the reference code samples for each partial correlation. The lookup is controlled by two lookup indexes. First, the code must be selected from 1 of 32 codes. This selection is constant through the convolution process and is established when the processing channel is configured to correlate for a particular satellite signal. The second index is a segment index between 1 and M. Each C/A code consists of 1023 chips, which are divided into M non-overlapping segments each consisting of K adjacent code chips. The lookup index identifies which code segment is needed. The output from the code lookup circuit is K chips comprising the segment. The selection process is controlled by Control/Address Logic414.
• Thecode extender409 takes as its input K chips of a segment and extends the segment into K×P code samples. The extension operation consists of converting each code chip into P identical code samples. The output from thecode extender409 forms the reference code input to vector multipliers402a-b. In the example, the output from the code extender is 66 samples made up of 33 unique values, each replicated twice.
• The architecture shown inFIG. 4 requires a clock that is substantially faster than the C/A code rate f_o. For example, if two samples per C/A code chip are used (P=2) and K and M are to be 33 and 31 respectively, achieving the full convolution requires performing 31 partial correlations for each shift of the signal shift register, which advances atrate 2×f_o. Typically, at least two clock cycles are required to read and writeRAMs110aand110b. Assuming two clock cycles, the minimum clocking rate required to achieve the full convolution is:
  f_clk=2×31×2×f_o=2×31×2×1.023 MHz≈127 MHz
  This rate is easily achievable in modern integrated circuit logic.
• It should be noted that the invention could also be used to compute a subset of the full convolution. In this case, fewer than M partial correlations are performed for each shift of the signal shift register. In this case, the total range of delays will be less than the P×1023 making up a full convolution. In particular if M₂partial correlations are performed, then M₂times K times P delay values are generated. The clocking rate to the processor is reduced by the ratio of M₂to M. Furthermore, the size of the RAMs is reduced by this ratio as well. Thus, this alternative may be useful in systems that do not have the computation or memory resources to process the full convolution.
• Other choices for K and M result allows further design tradeoffs to be made, however, since the prime factors of 1023 are 3, 11, and 31, the choices for K and M are limited. Reducing K is desirable since this reduces the size of the shift registers401aand401band the complexity of thevector multipliers402aand402b, but requires a larger M and therefore a large clocking rate. The choices for K are 3, 11, 31, 33, 93. These choices would require clocking rates of 1.39 GHz, 380 MHz, 135 MHz, 127 MHz, and 45 MHz respectively (always assuming P=2 and 2 clock cycles per partial correlation.) Based on the technology available at the time of the demonstration, the K=33 choice was made for one embodiment. With future technologies, the choice of K=11 and a clock rate of 380 MHz may become viable and would result in a further reduction of the logic complexity. Thus, the architecture has the desirable attribute of supporting optimized tradeoffs between speed and logic complexity.
• The sequencing of code segments is controlled bycontrol logic414. This control logic also identifies the correct addresses for theRAMs110a,110band113. As will be discussed below, the partial correlations are generated in a non-sequential order, thus the generation of RAM addresses is non-trivial.
• The operation of the circuits ofFIG. 4 can also be understood by reference to the flow diagram ofFIG. 5. Operation begins atstep501 with pre-loading of thesignal shift registers401aand401b. At this point, convolution processing can begin. Atstep502, a code segment is accessed for the particular partial correlation. Atstep503, the code segment is extended by the code extender to have P samples per C/A chip. Next, atstep504, the delay index and corresponding RAM addresses are computed. The delay index indicates which point of the full convolution will be updated by the partial correlation. As will be apparent from the example discussed in conjunction withFIG. 7, the delay index jumps around in a non-linear, but deterministic manner. The address computation is a function of the number of signal shifts and the code segment.
• Atstep505, the partial correlation is computed using thevector multipliers402aand402b. Atstep506, the result is accumulated into the signal RAMs at the location indicated by the delay index. Next atstep507, a check is made to determine whether the processing has reached the end of the coherent integration interval. If not, the method returns back to step502a, and repeats for the above steps for the next code segment.
• If, atstep507, the check indicates that partial correlations are complete for all code segments (e.g., 31 partial correlations), the method proceeds to step508. Atstep508, the signal registers401aand401bare shifted by one sample.
• The process then moves to step509, where a check is performed to see if the last shift encountered the end of the coherent integration interval. If not, the process cycles back to the start atstep502. If the check indicates the end of the coherent integration interval, then the method continues to step510, where the signal magnitude is computed bycomplex normalizer111. The result is added usingadder112 and stored in themagnitude RAM113. Next, atstep511, a check is made to determine if all magnitude accumulations have been performed. If so, the method completes atstep512. If not, processing continues by performing the next partial correlation atstep501.
• FIG. 6 andFIG. 7 illustrate, through a simplified example, how the invention utilizes partial correlations to accumulate a full convolution result. For clarity, these diagrams illustrate convolution of a very short length 6 code, as opposed to the length 1023 C/A code of a GPS signal. To further simplify the example, one sample per code chip is used, i.e. P=1.FIG. 6 illustrates convolution through a standard matched filtering approach, andFIG. 7 illustrates the identical convolution through the method of combining of partial correlations. The details ofFIG. 7 are helpful in understanding overall operation of the invention. Both methods generate identical convolution results.
• FIG. 6 shows the operation of a conventional matched filter for a length 6 signal. Operation begins at a moment in time indicated asshift 0. At this moment, 6 consecutive signal samples comprising an entire cycle of the signal are in thesignal shift register601. Individual samples are labeled with uppercase indices A, B, C, D, E, and F. Code samples for the entire length 6 code are held inreference register602 and are labeled with lowercase indices a, b, c, d, e, and f. At the time ofshift 0, a vector multiplication and add is performed to generate the correlation result forshift 0. Each signal sample is multiplied by a corresponding code sample and the results are summed to yield correlation result603.
• Next, thesignal shift register604 is advanced by one sample, as indicated byshift 1. The signal is periodic, so the new sample introduced at the left side of the register is identical to that shifted out to the right. The shifted contents of theregister604 are now samples with indices F, A, B, C, D, and E. The code is not shifted. The vector multiplication and addition now yields a correlation result605 forshift 1. This process of shifting continues for 5 additional shifts, at which point all 6 correlation results making up the full convolution are available.
• FIG. 7 illustrates how the same convolution result can be obtained through the method of partial correlations. As described, the invention requires that the code be factored into M segments of length K. In the simplified example ofFIG. 7, the length 6 code was factored into 3 segments oflength 2, i.e. K=2 and M=3. Operation begins at a moment in time indicated atshift 0. At this moment, two signal samples are held in thesignal shift register701. The signal samples are labeled with uppercase indices A and B. The 6 samples of the code are contained in 3 segments each oflength 2. Thefirst code segment702 contains 2 code samples labeled with lowercase indices a and b. The signal is held in place for 3 partial correlation operations, resulting in partial correlation results703a,703band703c. The first partial correlation result is created by a vector multiplication and addition between the contents of the signal register and the first code segment (segment 1). The second and third results are created by vector multiplications of the signal register with the second and third code segments respectively. Note that the signal register is held in place for a sufficient time for all three-vector multiplications to be performed, and that the code is not shifted during this time, rather different code segments are selected.
• The partial correlation results are accumulated into the memory according to thesignal paths705. For example, atshift 0, the partial correlation from the first code segment sums into thecorrelation result704. The partial correlation from the second segment sums into thecorrelation result706 forshift 2. The partial correlation from the third segment contributes to thecorrelation result708 forshift 4.
• After three partial correlations, the signal is shifted. At this stage, indicated asshift 1, the signal register contains samples F and A. Again, three partial correlations are generated with the same three code segments as before. The results from these partial correlations contribute tocorrelation results710,712,714 respectively forshifts 1, 3, and 5. The process continues for 4 additional signal shifts, at which time the full convolution result is available. As can be seen, the operation requires generating a total of 18 partial correlations that contribute to the 6 full results comprising the convolution.
• The architecture described byFIG. 7 illustrates two important properties of the invention. First, it is apparent that the full convolution was produced for a length 6 code using only a shift register and vector multiplication and addition unit oflength 2. This requires less circuitry than theFIG. 6 where these elements are of length 6. Second, inFIG. 7, the code samples are accessed in fixed segments that are the same for each shift, and each segment is a separate non-overlapping section of the code. Thus, a simple lookup or register scheme can be used to provide the code to the vector multipliers, as will be discussed further in reference toFIG. 8 andFIG. 9. These schemes require less circuitry than other architectures that might, for example, require large blocks of code bits to be made available in a more complex set of permutations. The invention also eliminates the need to provide code generation circuitry.
• FIG. 8 shows a block diagram of one embodiment of acode lookup circuit408. Table801 contains stored values for all 1023 bits of each of 32 codes, for example in read-only memory (ROM) or hard-wired logic. The table801 is organized as 32 sub-tables, one for each code. Each sub-table is further organized as M segments of length K where K×M=1023, and K and M are chosen as described previously.Multiplexer802 selects a particular code based on a select value. The output ofmultiplexer802 is a particular sub-table for the desired code.Multiplexer803 selects a particular segment based on a segment select value between 1 and M. The output of803 is aparticular code segment804, of length K, which contains code bits provided tocode extender409.
• It should be noted thatmultiplexer803 must be high speed in order to allow the code segment to be changed each partial correlation, i.e. every two clock cycles. For this reason, it is necessary that all code bits be pre-stored in table801, as opposed to being generated on the fly in the traditional manner of a code generator.
• The circuits ofFIG. 8 are intended to be illustrative. In practice, there are many different circuit designs that are functionally equivalent. In particular, the process of logic synthesis used in modern ASIC design will lead to a certain pattern of gates that achieves a behavior equivalent to that described above but not necessarily using multiplexers in the manner described.
• FIG. 9 shows a block diagram of an alternate embodiment of acode lookup circuit408. The 1023 code bits corresponding to a particular code are held in 1023 dual-directional shift registers901, organized as M rows of length K. The shift registers operate in two modes: a running mode, and a loading mode.
• In the running mode, eachregister901 is configured to shift its sample to the register above it in the next row, except for the top row of registers that shifts to the bottom row of registers. The shift directions for running mode are indicated by solid arrows within901. By clocking all the registers, rows of code bits will circulate, such that at any one time the top row contains one of M code segments of length K. This top row of bits is provided tocode extender409. The registers circulate rapidly, so that a different code segment is made available for each partial correlation.
• In the loading mode, each register is configured to shift its sample to the register next in its row, except for the last column of registers, which shift to the first column of registers in the row above. The shift directions for loading mode are indicated by dotted arrows within901. The left-handlower shift register904 is connected to codegenerator902. The code generator is a traditional code generator, capable of sequentially creating the 1023 code bits of a particular code based on a select value. When the code lookup circuit is configured for a particular code, the registers are placed in the loading mode, and the generator is used to generate the bits of the code, which then clock through the registers. After all bits have been clocked through, the code will reside in the registers as M segments of length K. The circuit is then ready for use in the running mode.
• FIG. 10 depicts a block diagram of an alternative embodiment of aGPS receiver1000 capable of operating in multiple modes of resolution. TheGPS receiver1000 has aconvolution processors1009 that operate either in a standard resolution mode or a high-resolution mode. Moreover, digital signal samples in standard resolution mode are spaced ½ of a C/A code chip apart (i.e., P=2). Digital signal samples in the high-resolution mode are spaced ⅕ of a C/A code chip apart (i.e., P=5). As such, theCode NCO1008 and thedecimation circuit1007 operate at a plurality of sampling rates. Those skilled in the art can readily devise other values for the sample spacing and understand that the invention can operate in more than two modes of resolution.
• The embodiments ofFIG. 10 has similar components as those depicted inFIG. 1. Components having the same reference number as those inFIG. 1 operate as described above with respect toFIG. 1. Components such asdecimation circuit1007,Code NCO1008,convolution processor1009 and mode selection processor1002 operate as described below to facilitate the use of a plurality of resolution modes.FIG. 10 further comprises a mode selection processor1002. The mode selection processor1002 processes received signals to determine whether the processing channel1004 should operate in the high or standard resolution mode.
• As with the embodiment shown inFIG. 1,FIG. 2 depicts waveforms generated by the components ofFIG. 10. Although the waveforms depicted inFIG. 2 are illustrative of operation in standard mode, operation in high-resolution mode would generate waveforms of similar character. Specifically, if P=5 in high-resolution mode, the total number of points in the full convolution would be 5115 rather than 2046. However, as will be described more fully below, only a portion of the convolution is generated in the high-resolution mode.
• It should be noted that in standard resolution (i.e., P=2), the least square estimation uses only 4 points over the width of the correlation, as shown by thewidth303 of the triangle base inFIG. 3. In the presence of noise, this limits the accuracy of the curve fitting process. Furthermore, in many cases the center of the triangle (i.e., the point of maximum correlation) will reside between the observed correlation values. In this situation, the observed correlation, and hence the observed signal to noise ratio, is significantly lower than the case where the peak of the triangle is close to an observed point. The high-resolution mode improves accuracy and signal to noise ratio by including many, finely spaced points in the least squares estimation process.
• Specifically,FIG. 11 shows the details of an accumulated magnitude waveform when the same correlation as shown inFIG. 2 is processed in high-resolution mode. Thegraph1100 shows the magnitude of the convolution in the vicinity of a peak212₃corresponding to the time delay of the signal processed in high-resolution mode. Points on thecode chip axis210 are spaced at an interval equal to the C/A code chip length divided by P, where P is the ratio of the signal-sampling rate to f_o, the C/A code chipping rate. In the high-resolution example, P=5, so the points are spaced at ⅕ chip intervals, or approximately 200 ns. (This spacing in time corresponds to a range difference of approximately 60 meters). In order to achieve high accuracy pseudorange measurements, the output of the convolution is further processed, typically in theCPU114.
• As with standard resolution processing, there are numerous interpolation techniques that can be used to estimate the true time delay, using discrete correlation values provided by the convolution. One embodiment uses a least squares estimation technique to identify parameters of a signal that best fits the noisy data. The correlation response takes on the form of a raisedtriangle1102. Thewidth1103 of thetriangle1102 is exactly 2 C/A code chips, which corresponds to 11 sample points (for the P=5 case). Theheight1104 of the base of thetriangle1102 is the magnitude of the noise in the convolution for time delays not corresponding to the signal. The magnitude of this noise can be estimated from the data or pre-calculated based on design parameters, such as the amplifier noise figure, cable and filter loss, and system temperature. Thepeak1105 of thetriangle1102 and thecenter1106 are unknowns corresponding to the signal magnitude and time delay. The least squares method can be used to estimate these two parameters so as to fit the noisy data points to a triangle with a particular peak and center.
• One benefit of the high-resolution mode over the standard resolution mode is that the raised triangle correlation response is sampled at more than twice as many points. As understood by those skilled in the art, the accuracy of the fitting process depends on the number of values used in the estimation. Furthermore, the ratio of the peak of the triangle to the base of the triangle is increased in high-resolution mode. This indicates an improvement in signal to noise ratio, due in part to correlation points being available close to the point of maximum correlation. Thus, high-resolution mode can be used to reliably identify and measure correlation peaks that may not be properly discernable in standard mode. This can be very advantageous when processing low energy signals, such as those signals received indoors from GPS satellites.
• As will be described further with regard toFIG. 12, the two modes of operation are achieved by dynamically changing the values of the parameters P, K, and M to obtain the desired resolution. In one embodiment, P=2 (i.e., two samples per C/A code chip) in standard mode and P=5 (i.e., five samples per C/A code chip) in high-resolution mode. The completed correlation points are spaces at 1/P chips apart, thus it is clear that the larger the value of P, the finer the resolution. As discussed above, K is chosen as a design parameter and is a factor of 1023. For simplicity, the remainder of the discussion focuses on one particular embodiment where P=2 and K=33 in standard mode; and P=5 and K=11 in high-resolution mode.
• FIG. 12 illustrates a block diagram of another embodiment of aconvolution processor1009 suitable for operation of the invention in multiple modes of resolution. For simplicity, only the I channel of processing is shown, but it is clear that the Q channel comprises identical processing elements. In the present embodiment, theconvolution processor1009 comprises ashift register1201, avector multiplier circuit1202, anadder1203, acode extender1209, and a code lookup table1208. Thecode extender1209 further comprises astandard code extender1204 and a high-resolution code extender1205. The I signal from thedecimation circuit1007 is coupled to theshift register1201.Shift register1201 is of variable length since the value of P×K changes with regard to standard and high-resolution modes. Specifically,shift register1201 must hold 66 samples in standard mode and 55 samples in high-resolution mode. Thus, shift-register1201 comprises 66 elements to support both lengths. In high-resolution mode, the last 11 elements are disabled.
• Signals advance throughshift register1201 at the rate of Pf_oas timed by thecode NCO1008 for either standard mode or high-resolution mode. The signals remain in place for many clock cycles so that a series of partial correlation operations can be performed. Specifically, a total of M partial correlations are performed, where M=1023/K. For the present example, in standard mode, M=31, and in high-resolution mode, M=93. Each partial correlation consists of a fast vector multiply and add operation between the contents ofshift register1201 and a segment of the code containing P×K code samples. The operation is performed viavector multiplier1202, which comprisesmultipliers1210 and asummer1212. Similarly to shiftregister1201,vector multiplier1202 is of a variable length to support both standard and high-resolution modes of operation.
• The correlation operation consists of multiplying each of the P×K signal samples inshift register1201 by P×K code samples (formed by extending the code samples with code extender1209), and summing the results insummer1212. As discussed above, mathematically, this operation is referred to as the inner product. The results of the vector multiply and add are accumulated byadder1203 and processed by the convolutionresults processing circuits400 in the same manner as discussed above with regard toFIG. 4.
• Code lookup table1208 generates the reference code samples for each partial correlation and is organized to supply code segments for both standard and high-resolution modes. First, the code must be selected from 1 of 32 codes. This selection is constant through the convolution process and is established when the processing channel is configured to correlate for a particular satellite signal. The second index is a segment index between 1 and M. In the present example, the number of samples per segment is 33 in standard mode and 11 in high-resolution mode. To support both modes, the code lookup table1208 comprises 93segments1207 of 11 chips. The 93segments1207 are further organized into 3 blocks1206a-cof 31 segments each. The output of each group1206a-c(an 11 chip segment) is coupled to thecode extender1209.
• In standard mode, the output segment of each group1206a-cis combined to form a wide segment having 33 chips. Specifically, a segment selection is made between 1 and 31. The segment selection and the code selection are used tomultiplex 3segments1207 of 11 samples from groups1206a-c. Thesegments1207 are concatenated to form a 33 sample wide segment, which is the input to thestandard code extender1204. In high-resolution mode, the segment selection runs from 1 to 93. Only one of thesegments1207 is chosen from groups1206a-c. The 11sample segments1207 are then coupled to the high-resolution code extender1205.
• In standard mode, thestandard code extender1204 is active, which extends the 33 sample wide code segments to 66 samples. In high-resolution mode, the high-resolution code extender1205 is active, which extends the 11 sample code segments to 55 samples. Although thecode extender1209 is shown as comprising astandard code extender1204 and a high-resolution code extender1205, it is understood by those skilled in the art that they could be functionally combined into asingle code extender1209.
• Clock rates change between standard and high-resolution modes. For example, in standard mode, as discussed with regard toFIG. 4, a clocking rate of approximately 127 MHz supports performing a full convolution in real time for the case where P, K, and M are 2, 33, and 31 respectively and two clock cycles are needed per RAM cycle. In the present embodiment, the high-resolution mode parameters P, K, and M are 5, 11, and 93 respectively. Thus the clocking rate to generate the full convolution is:
  f_clk32 5×93×2×f_o=5×93×2×1.023 MHz≈952 MHz
  Since this rate is difficult to achieve in modern integrated circuit logic, and since the size of the RAMs would grow from 2046 samples to 5115 samples, it may be desirable to generate less than the full convolution when operating in high-resolution mode in order to reduce cost and complexity of theconvolution processor109.
• Therefore, in one embodiment, less than the full convolution is computed in high-resolution mode by reducing the number of partial correlations performed for each shift of the input signals. As will be describe below with regard toFIG. 13, the partial correlations are selected so that points of correlation are generated for a specific region of interest within the full convolution. In this alternative embodiment, the clock rate required is:
  f_clk′=5×L×2×f_o
  where L, a number smaller than M, represents the number of partial correlations per input cycle shift. Thus, the total number of correlation points generated is P×K×L. For example, in one embodiment L=12 and P×K×L=660, or about ⅛ of a C/A code epoch. In this example, the clocking rate required is:
  f_clk=5×12×2×1.023 MHz≈123 MHz
  a value that is commensurate with the clocking rate in standard mode.
• Since only a subset of the full convolution is generated, it is necessary to select a portion of interest containing the signal peak.FIG. 13 depicts a flow diagram of a method ofoperation1300 of the invention for computing multiple correlation resolutions. Themethod1300 begins atstep1302. Atstep1304, checks are made to determine whether sufficient external aiding information exists that enables direct use of high-resolution mode. This aiding information could take several forms, including estimated pseudoranges for satellites of interest and an estimate of local clock bias. If the aiding information is sufficiently accurate, the method transitions to high-resolution mode atstep1310. Often times, however, the local clock bias is unknown. For example, estimated pseudoranges may be available from stored ephemeris and a last known receiver position, but, unless the receiver maintains precise timekeeping or is externally synchronized, the local clock bias will be unknown. If there is not sufficient external aiding information, pseudorange measurements are made in standard mode atstep1306. Atstep1306, a full convolution is obtained using a standard resolution. After one or more measurements are made in standard mode, the local clock bias is estimated atstep1308. This step optionally includes an estimation of time tag error. Themethod1300 then transitions into high-resolution mode atstep1310. At this point, the signal delays can be bounded to a plurality of windows bracketing the correlation peaks at the standard resolution, which is computed atstep1312. After the range windows are computed, themethod1300 makes pseudorange measurements in the high-resolution mode (i.e., using the high resolution correlation peaks) atstep1314. Themethod1300 ends atstep1316.
• Themethod1300 allows the high-resolution mode to be used beneficially notwithstanding the constraint that only a partial convolution is computed. As long as at least one satellite is detected in standard mode, high-resolution mode can be invoked. In high-resolution mode, the sensitivity of the correlation processing is enhanced, which enables additional satellites to be detected. Furthermore, as described above, the accuracy of all measurements is improved in high-resolution mode.
• In another embodiment, the present invention mitigates multipath interference in the received satellite signal during signal correlation. In general, the raised triangle correlation response shown inFIGS. 3 and 11 will be distorted when the received signal contains multipath interference.FIG. 14 shows correlation responses similar to that ofFIG. 11, but affected by multipath interference. In particular,FIG. 14A shows an accumulated magnitude of acorrelation response1406, where the direct satellite signal is stronger than a reflected satellite signal (i.e., a multipath signal).FIG. 14B shows an accumulated magnitude of acorrelation response1408, where the direct satellite signal is weaker than a reflected satellite signal. InFIGS. 14A and 14B,axis1402 represents signal strength andaxis1404 represents the sample number.
• As shown inFIGS. 14A and 14B, points on thesample axis1404 are spaced at an interval equal to the C/A code chip length divided by P, where P is the ratio of the signal-sampling rate to f_o, the C/A code chipping rate. In the high-resolution example, P=5, so the points are spaced at ⅕ chip intervals, or approximately 200 ns (approximately 60 meter range difference). The presence of a multipath interference in the received satellite signal causes the formation of two or more peaks in the correlation response. When a direct signal from a satellite is received, thefirst peak1410 will correspond to the correlation between the recovered PRN code from the direct signal and the generated replica code. Thesecond peak1412 corresponds to the correlation between a recovered PRN code from a reflection of the satellite signal and the generated replica code. The reflected signal arrives later in time than the direct signal, and thus thesecond correlation peak1412 is later in time than thefirst correlation peak1410.
• InFIG. 14A, thefirst correlation peak1410 has a higher magnitude than thesecond correlation peak1412. That is, the direct signal is stronger than the reflected signal.FIG. 14B is similar toFIG. 14A, with the exception that thefirst correlation peak1410 is lower in magnitude than thesecond correlation peak1412. Thus, inFIG. 14B, the direct signal is weaker than the reflected signal.
• As apparent fromFIGS. 14A and 14B, the correlation response that best fits the data does not necessarily take the form of a raised triangle. In most cases, an attempt to fit a triangle to thecorrelation functions1406 and1408 will result in a center that does not correspond to the proper time delay. That is, the center of the triangle will be offset from the true center indicated by dashedline1414. Dashedline1414 corresponds to thecorrelation peak1410 of the direct satellite signal, and thus indicates the correct time delay. Furthermore, conventional tracking correlators, such as early-late (E-L) correlators, also fail to converge on the correct time delay due to thesecond correlation peak1412.
• As described below, the present invention mitigates the effect of multipath interference by obtaining a sufficient number of samples to approximate the entire shape of the distorted correlation response. Given the entire shape of the correlation response, the invention can locate the first maximum in the correlation response to determine the correct correlation peak. Employing a large number of samples allows the invention to use open loop estimation for determining the correct time delay and signal magnitude, obviating the need for a closed loop correlator, such as an E-L correlator. In addition, the algorithm operates on the accumulated magnitude or power of the correlation response (the magnitude is the square root of the power). This allows for operation at much lower signal-to-noise ratios than would be possible with conventional coherent correlators. Thus, the present invention allows for accurate correlations in weak signal environments with many multipath signals, as may be encountered in urban environments.
• Specifically,FIG. 15 depicts a flow diagram showing amethod1500 for mitigating multipath interference in a digital signal correlator. Themethod1500 can be best understood with simultaneous reference toFIG. 14. Themethod1500 can be implemented as software executed by the CPU114 (FIG. 1). Themethod1500 begins atstep1502 and proceeds to step1506, where the received satellite signal is processed in high-resolution mode. As described above, high-resolution mode corresponds to ⅕ chip intervals or 200 ns sample spacing. In another embodiment, a “super” high-resolution mode can be employed having 1/10 chip intervals or 100 ns sample spacing. Those skilled in the art will appreciate that other values for P can be used for the high-resolution mode.
• Alternatively, the received satellite signal can be processed at a first resolution (e.g., ½ chip intervals) and then processed at a second higher resolution (e.g., ⅕ chip intervals) when the receiver enters an area where there is multipath interference. Such multipath interference can be detected atoptional step1504 before high-resolution processing atstep1506. If multipath interference is not detected, the standard peak measurement process can be used as described above with respect toFIG. 3. If multipath interference is detected, the method proceeds to step1506.
• In one embodiment, theCPU114 can detect that a correlation response has been affected by multipath signals by measuring the slope of the ascendingedge1416 of the correlation response and the slope of the descendingedge1418 of the correlation response. If the angles of these slopes are not substantially symmetrical, the correlation response is affected by multipath signals. Multipath signals of differing amplitude and delay result in a significantly slower descending slope than the ascending slope (or significantly faster if the reflected path is in opposition of phase).
• In another embodiment, theCPU114 can detect that correlation response has been affected by multipath signals by measuring the width of the correlation response. That is, theCPU114 can measure the distance between the intersection of the ascendingedge1416 with amean noise level1420, and the intersection of the descendingedge1418 with themean noise level1420. If this distance is greater, or much narrower than two C/A chips, the correlation response is likely affected by multipath signals. The mean noise level can either be computed as part of the processing of the correlation response, or the mean noise level can be estimated a-priori (e.g., estimated based on settings such as the amount of averaging).
• In yet another embodiment, a plurality of processing channels104 are used with a single satellite signal, with the timing of the correlations staggered to provide high resolutions. In any case, the output of the process ofstep1506 is a correlation response generated in high-resolution mode.
• Atstep1508, the values representing the magnitude of the correlation response are formed. Atstep1510, theCPU114 selects the earliest peak in the correlation response as the correct peak. Themethod1500 ends atstep1512.
• In one embodiment, theCPU114 selects the earliest peak by determining the maxima for the correlation function. As used herein, the term “maxima” means both local and absolute maximum points on the correlation function. In high-resolution mode (e.g., P=5), the correlation function contains enough samples to approximate the entire shape thereof. Given the entire shape of the correlation response, theCPU114 can differentiate between the correlation peak resulting from the direct signal and correlation peaks resulting from reflected signals. In one embodiment, theCPU114 determines the maxima of the correlation response using differential analysis. Specifically, theCPU114 computes the first and second derivatives of the correlation response and solves for the maxima in a well know manner. For example, in reference to the correlation responses shown inFIGS. 14A and 14B, points1410 and1412 are maxima thereof. TheCPU114 analyzes the computed maxima of the correlation response to determine which of the maxima is the earliest in time. As described above, the earliest peak is the correlation peak corresponding to the direct satellite signal. Thus, the earliest computed maximum point on the correlation response is the correlation peak corresponding to the direct satellite signal and is thus the correct correlation peak. Given the correct correlation peak, theCPU114 determines the correct time delay and magnitude.
• In alternative embodiments, theCPU114 can use the following algorithms to select the earliest peak in the correlation response. The following algorithms account for the fact that the width of the correlation response is larger than a standard correlation response by an amount equal to the direct peak delay and the reflected peak delay.
• In one algorithm, theCPU114 can detect multiple peaks by looking for “dips” in the correlation response (i.e., samples in which the power is not continually increasing before the maximum power). Each dip represents a local or absolute minimum and indicates multiple peaks in the correlation response. The maximum occurring before the first dip (i.e., the earliest maximum) is the correct peak in the correlation response and is thus the correct measurement of the delay.
• In another algorithm, theCPU114 can determine the slope of the correlation response on theascending edge1416. The true correlation peak is 1 chip away from the intersection of that slope with themean noise level1420 of the correlation vector. This allows the selection of the correct delay in situations where the above algorithms cannot detect multiple peaks due to close proximity of the reflected peak, or due to the presence of multiple reflected peaks.
• While the foregoing is directed to the preferred embodiment of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims (20)

1. A method for selecting a signal processing resolution in a satellite signal receiver comprising:

receiving a first signal at a satellite signal receiver;

processing the first signal at a first resolution;

detecting a multipath condition when processing the first signal; and

in response to the multipath condition, processing a second signal at a second resolution, where the second resolution is higher than the first resolution.

2. The method ofclaim 1 wherein the first and second signals are satellite signals transmitted by at least one satellite in a satellite positioning system.

3. The method ofclaim 2 wherein the satellite positioning system is at least one of a Global Positioning System (GPS), Galileo, or GLONASS.

4. The method ofclaim 1 wherein the first signal processing step further comprises performing signal correlation to generate a correlation result.

5. The method ofclaim 1 wherein the detecting step further comprises:

measuring a width of a correlation result;

comparing the width to a predetermined value; and

if the width is greater than the predetermined value, indicating a multipath condition.

6. The method ofclaim 5 wherein the measuring step further comprises:

measuring a slope of a ascending edge of the correlation result;

measuring a slope of a descending edge of the correlation result;

measuring a mean noise level;

computing a first intersection of the ascending edge and the mean noise level;

computing a second intersection of the descending edge and the mean noise level;

computing the width of the correlation result as a distance between the first and second intersections.

7. The method ofclaim 6 wherein the predetermined value is two chips of the satellite signal.

8. The method ofclaim 1 wherein the detecting step further comprises:

measuring a slope of a ascending edge of the correlation result;

measuring a slope of a descending edge of the correlation result;

comparing the ascending slope to the descending slope; and

if the ascending slope differs from the descending slope, indicating a multipath condition.

9. The method ofclaim 1 wherein the first resolution is a one-half chip interval and the second resolution is a one-fifth chip interval.

10. The method ofclaim 1 wherein the second signal processing step further comprises

performing signal correlations to generate a second signal correlation; and

selecting an earliest peak in the second signal correlations.

11. Apparatus for processing a satellite signal comprising:

a satellite signal receiver for receiving a first signal;

a correlator for processing the first signal at a first resolution and for processing a second signal at a second resolution, where the second resolution is higher than the first resolution;

a detector, coupled to the correlator for detecting a multipath condition when processing the first signal and switching the correlator to the second resolution upon detecting the multipath condition.

12. The apparatus ofclaim 11 wherein the first and second signals are satellite signals transmitted by at least one satellite in a satellite positioning system.

13. The apparatus ofclaim 12 wherein the satellite positioning system is at least one of a Global Positioning System (GPS), Galileo, or GLONASS.

14. The apparatus ofclaim 12 wherein the correlator performs signal correlation to generate a correlation result.

15. The apparatus ofclaim 12 wherein the detector further comprises:

a processor for measuring a width of a correlation result; comparing the width to a predetermined value; and if the width is greater than the predetermined value, indicating a multipath condition.

16. The apparatus ofclaim 15 wherein the processor measures the width of the correlation result by measuring a slope of a ascending edge of the correlation result; measuring a slope of a descending edge of the correlation result; measuring a mean noise level; computing a first intersection of the ascending edge and the mean noise level; computing a second intersection of the descending edge and the mean noise level; and computing the width of the correlation result as a distance between the first and second intersections.

17. The apparatus ofclaim 16 wherein the predetermined value is two chips of the satellite signal.

18. The apparatus ofclaim 11 wherein the detector measures a slope of a ascending edge of the correlation result; measures a slope of a descending edge of the correlation result; compares the ascending slope to the descending slope; and if the ascending slope differs from the descending slope, indicating a multipath condition.

19. The apparatus ofclaim 11 wherein the first resolution is a one-half chip interval and the second resolution is a one-fifth chip interval.

20. The apparatus ofclaim 11 wherein the correlator performs signal correlations to generate a second signal correlation; and selects an earliest peak in the second signal correlations.

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